Nobody knows? It gets worse for me...
I'm totally confused now. Everybody seems to agree that the Bosch part no. is ZR6SII3320, but I've been given three different MB part numbers: A0041598103, A0242140521, and A0041596403; the last is what was fitted by MB Cambridge in November 2015, 40K miles ago.
MB Newcastle, from whom I will be getting my plugs, says that A0041598103 is the current plug for my car, but of course either or both of the other part numbers could be superceded. If MB Cambridge fitted the wrong plugs, I might ask them for a contribution to the cost; the correct plugs should not be giving trouble so soon.